To add a nick/address to the User Levels, you can use the /auser command.
/auser <level> <nick|address>
/auser 20 Snerf
/auser 30 *!*@host.com

To remove a nick/address from User Levels, you use the /ruser command.
/ruser [level] <nick|address>
The [level] value is optional.
/ruser Snerf – Removes all occurences of Snerf with any level
/ruser 20 Snerf – Removes Snerf which has level 20

To remove nick/addresses pertaining to just user levels, use the /rlevel command.
/rlevel <level>
/rlevel 20 – Removes all nick|addresses with level 20